1. Field of the Invention
This invention relates generally to high quality short period bulk domain inversion structures (gratings), and more particularly to high quality short period bulk domain inversion structures (gratings) that are fabricated in substrate materials such as MgO doped congruent lithium niobate using electric field poling.
2. Description of the Related Art
Quasi phase matching (QPM) is an efficient way to achieve nonlinear optical interactions. The approach was first proposed by Bloembergen et al. (U.S. Pat. No. 3,384,433), using a domain inversion grating structure to achieve QPM. Such a domain grating structure can be usefully realized in an optically transparent ferroelectric material, such as lithium niobate (LiNbO3), lithium tantalate (LiTaO3) and potassium titanyl phosphate compounds (KTP). There are many different ways to achieve inverted domain structures in these materials.
A periodic poled material structure can be grown directly within the material by modifying a parameter during the growth process, such as temperature, or a dopant concentration. Ming et al. (“The growth striation and ferroelectric domain structures in Czochralski grown LiNbO3 single crystals,” Journal of materials Science, v11, p. 1663, 1982.) used variation of temperature, growth rate and solute concentration during Czochralski growth to create a periodic structure in Lithium Niobate. Laser heated pedestal growth is disclosed in U.S. Pat. No. 5,171,400 by Magel et al. from Stanford University. This method can produce gratings with periods as short as 6 μm and 4 μm, but it is difficult to grow long lengths and curvature of the domains limits the lateral dimensions and efficiency.
Impurity doping or material removal in some ferroelectric materials (such as lithium niobate and KTP) can result in domain inversion. In lithium niobate, periodic domain inversion gratings can be achieved through high temperature processes such as titanium indiffusion, lithium outdiffusion (in air, or enhanced with surface layers of SiO2 and MgO) or proton exchange. A mechanism for the domain inversion was proposed by one of the present inventors, based on space charge field of impurity gradients (Huang et al. “A discussion on domain inversion in LiNbO3”, Appl. Phys. Lett. v65. p. 1763, 1994). Byer et al. at Stanford University (U.S. Pat. No. 5,036,220) demonstrated a waveguide frequency converter wherein the domain structure was created using titanium indiffusion in lithium niobate.
Due to the typically shallow impurity diffusion depths, the inverted domains are also typically shallow and generally triangular or semicircular in depth in lithium niobate.
A high voltage may be used to generate domain inversion at room temperature. Papuchon (U.S. Pat. No. 4,236,785) demonstrated patterned electric field in-plane poling on lithium niobate to achieve waveguide quasi-phasematched nonlinear interactions. Short period domain inversion in Z-cut congruent lithium niobate (CLN) was first demonstrated by Yamada at Sony in 1992 (U.S. Pat. No. 5,193,023) but the described process suffered from limitations in the material thickness and high instances of destructive electrical breakdown. Since this first report many different techniques of applying the electric field have been demonstrated, generally enabling electric field induced domain inversion to be achieved at or near to room temperature, in contrast to the methods of impurity diffusion. Approaches include the use of patterned metal electrodes, patterned insulators with liquid electrodes (e.g., U.S. Pat. No. 5,800,767 and U.S. Pat. No. 5,519,802) and corona discharge (Harada et al. “Bulk periodically poled MgO:LiNbO3 by corona discharge method”, Appl. Phys. Lett V 69, #18, p2629, 1996, Fuji Photo Film Co Ltd). The common feature of all of these approaches is the creation of a localized electric field modulation (or patterned electric field) on one face of the substrate.
Bombardment with a high energy electron beam can be used to induce bulk domain inversion in congruent lithium niobate at room temperature as demonstrated by Yamada at Sony (Yamada et al. “Fabrication of periodically reversed domain structure for SHG in LiNbO3 by direct electron beam lithography at room temperature,” Elect. Lett. Vol 27 p. 828, 1991), without the use of an applied voltage. Ito et al. also performed electron beam writing of domain gratings in lithium niobate (Ito et al., “Fabrication of periodic domain grating in LiNbO3 by electron beam writing for application of nonlinear optical processes” Elect. Lett. Vol 27 p. 1221, 1991). The high energy electrons incident on the substrate penetrate the surface and are trapped inside the substrate. These localized trapped electrons in the material result in localized high electric field that causes domain inversion. Earlier work by Keys et al. (“Fabrication of domain reversed gratings for SHG in lithium niobate by electron beam bombardment”, Electronics Letters, V26, #3 p 188, 1990) used a mask to pattern the bombardment of a high energy electron beam on congruent lithium niobate and, combined with an elevated temperature and a small applied voltage, provided patterned domain inversion.
In essence, all the methods described above are electric field poling. The orientation of the internal dipole moment is reversed under the influence of the local and global electric field. In direct growth, and impurity diffusion approaches the electric field is generated from a temperature gradient, or a dopant gradient. With electron beam bombardment the electric field is created by the trapped electrons injected into the substrate from a high energy beam.
Early work in electric field poling for QPM applications concentrated largely on congruent lithium niobate since this is by far the most widely available nonlinear optical material and also one of the most versatile, with a transparency range from about 400 nanometers (nm) to 5 microns (μm) in wavelength. However, as applications have come to be developed for the visible spectrum, the large numbers of defects in the congruent crystal structure, together with trace impurities incorporated during the growth process, give rise to a property called photorefractivity. The photorefractive effect is caused by the directional drift of photo-excited charges generated by absorption of visible and ultraviolet (UV) light within the material, which creates a space-charge electric field. The space-charge electric field leads, via the electro-optic effect, to a refractive index change which distorts the optical beam passing through the crystal. In order to be used in applications using or generating visible light, congruent lithium niobate needs to be doped with about 5% MgO, as shown by Bryan et al. (“Increased optical damage resistance in Lithium Niobate,” Appl. Phys. Leff. V44. p 84, 1984) to overcome the effects of structural defects and eliminate the photorefractive effect.
However the MgO dopant in MgO:CLN brings an even bigger challenge in realizing periodic domain structures. Many groups of researchers around the world have been working on electric field poling of MgO:CLN. For example, corona poling was attempted by Fuji (Harada et al “Bulk periodically poled MgO—LiNbO3 by corona discharge method” Appl. Phys. Lett. V69 p 2629); the use of elevated temperatures was attempted by Mitsubishi Cable (U.S. Pat. No. 6,565,648), and Matsushita (Mizuuchi et al. “Electric field poling in Mg doped LiNbO3”, Jnl Appl Phys, V96, #11, 2004, Mizuuchi et al “Efficient second harmonic generation of 340 nm light in a 1.4 μm periodically poled bulk MgO:LiNbO3”, Jpn J Appl Phys V42, p 90-91, 2003); ultra-violet light and laser light energy assisted poling has been attempted by several other groups (Muller et at “Influence of ultraviolet illumination on the poling characteristics of lithium niobate crystals” Apl Phys Lett V83 #9 p 1824 2003, Valdivia et al “Nano scale surface domain formation on the +Z face of lithium niobate by pulsed ultraviolet laser illumination,” Appl Phys Lett V86 2005, Fujimura et al. “Fabrication of domain inverted gratings in MgO:LiNbO3 by applying voltage under ultraviolet irradiation through photomask at room temperature”, Elect Lett V39 #9 p 719 2003, Dierolf et al “Direct write method for domain inversion patterns in liNbO3”, Apl Phys Lett V84 #20 p 3987 2004). However, short-period-domain-grating structures have not been achieved at room temperature in a reliable and repeatable manner.
Part of the difficulty in poling MgO:CLN is the observation that there is current flow through the substrate other than the poling displacement current during the poling process. This current flow results in preferential growth of domains which are formed early in the poling process and disrupts the domain seeding uniformity and therefore the uniformity of the final grating pattern.
It is also found that the domain wall boundary in Mg doped CLN seems to be aligned less rigidly along the crystal axis than in the undoped CLN material. Since the inverted domain structure does not strictly follow the crystal structure, it is fundamentally challenging for the inverted domain to propagate through the entire thickness of the substrate while maintaining the lateral dimensions of the masking pattern applied on one surface of the substrate.
Accordingly, there is a need to provide an improved domain inverted grating device with high efficiency and high resistance to photorefractive effects and a fabrication method able to control the domain growth through the bulk of the crystal for short period domain inversion gratings for applications in high power visible light generation.